1. Field of the Invention
The present invention relates to a method of manufacturing a piezoelectric element such as a surface acoustic wave device, and more specifically, the present invention relates to an improvement to a method of manufacturing individual piezoelectric elements by dicing a wafer having electrodes, which include Al or Al alloy, on a piezoelectric substrate.
2. Description of the Related Art
When manufacturing piezoelectric elements such as a surface acoustic wave device, a mother wafer is first prepared and then is diced to obtain individual piezoelectric elements in order to improve mass productivity. More specifically, electrodes made of Al or Al alloy are formed on a piezoelectric substrate consisting of a piezoelectric material to form a wafer, and the individual piezoelectric elements are obtained by dicing the wafer.
When dicing the wafer, a method for dicing the wafer while cooling the wafer with cutting water has been popular. However, fragments and cutting dust from the piezoelectric material are generated from the piezoelectric substrate during the dicing, which results in the fragments of the piezoelectric material being adhered to surfaces of electrodes. In addition, a portion of the piezoelectric material is dissolved in the cutting water, which corrodes the electrodes. As a result, the characteristics of the piezoelectric elements produced according to this method are degraded.
In order to solve the above-described problem, a protective film such as a passivation film consisting of SiO2 is disposed on a surface having the electrodes of the wafer formed thereon before dicing occurs.
However, in the above-described method for forming the protective film, problems occur in that an unnecessary process for forming a protective film for the finished piezoelectric elements must be added, a manufacturing process becomes complicated, and the cost is increased.
In addition, even when the protective film is formed, the electrical characteristics of the obtained piezoelectric elements are sometimes lower than design values, or the non-defective ratio is not sufficiently high, possibly due to the adhesion of powders of the piezoelectric material generated by dicing the wafers.
Further, when using a wafer having electrodes thereon by forming a metal film consisting of an Al alloy on the piezoelectric substrate and etching the metal film with plasma using a halogen-containing gas such as CF4 and Cl2, or a wafer having a frequency that is trimmed by etching the substrate after forming the electrodes, a halogen element such as Cl and F works as an oxidizing agent, and a problem occurs in that the electrodes are corroded.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of manufacturing a piezoelectric element that produces piezoelectric elements having excellent characteristics and with an excellent non-defective ratio without forming any protective films before dicing.
According to a preferred embodiment of the present invention, a method of manufacturing a piezoelectric element includes the steps of forming an electrode including an Al alloy on a piezoelectric substrate made of a piezoelectric material, forming a piezoelectric element by exposing the electrode to plasma using a halogen-containing gas, and dicing the wafer while cooling the wafer with cutting water, wherein the cutting water includes a compound for reacting with Al to form a protective film on a surface of the electrode.
At least one kind of phosphoric acid and phosphate is preferably used for the compound that is included in the cutting water, and at least one material selected from the group consisting of calcium hydrogenphosphate, calcium dihydrogenphosphate and sodium hydrogenphosphate is preferably used for the phosphate.
In addition, the first and second preferred embodiments can be preferably used when a surface acoustic wave device is manufactured as the piezoelectric element.